This invention relates to a biomass gasifier.
Combustible gas can be obtained from agricultural crop waste and other biomass by a process known as gasification. FIG. 1 illustrates a conventional downdraft biomass gasifier, based on the disclosure in U.S. Pat. No. 6,647,903. The biomass is typically cellulose based and contains, in addition, volatile organic compounds (VOCs), tars and resins. Ground or chopped biomass is placed in a hopper 2 at the top of a gasification chamber 4 having a restricted outlet 8 at the bottom of the chamber and is ignited. A blower 10 draws air into the gasification chamber to support combustion of the biomass. The blower is controlled so that the rate of supply of oxygen is not sufficient for complete combustion of the biomass. As the biomass is heated, water and volatile compounds are released from the biomass, which forms a smoldering char composed primarily of carbon. The volatile compounds are oxidized, supplying heat to the biomass. Carbon in the char reacts with oxygen in reactions that produce carbon dioxide and carbon monoxide:C+O2→CO2+heat2C+O2→2CO+heat  (1)
Subsequently, carbon dioxide reacts with carbon in the char to produce carbon monoxide in a highly endothermic carbon reaction:Heat+C+CO2→2CO  (2)
Gasification thus produces a gas that contains CO and can be used as fuel. The fuel gas is discharged at the pressure side of the blower 10. Incombustible ash is discharged downward from the gasification chamber through a char tube 12 that terminates in an ash pit. The ash that accumulates in the ash pit partially blocks the char tube and prevents excessive air entering the gasification chamber. As ash is removed from the vicinity of the lower end of the char tube, more char and ash can enter the char tube at its upper end, permitting fuel to enter the gasification chamber from the hopper.
The conventional downdraft gasifier shown in FIG. 1 is subject to disadvantage due to bridging of the fuel and char in the gasification chamber, preventing proper feeding of biomass to the zone at which the carbon reaction occurs. Further, in operation of the conventional downdraft gasifier, flow of gas through the char tube may create channels such that the gas passes through the char tube too rapidly to permit the carbon reaction (2) to occur. In addition, the composition of gas delivered to the carbon reaction zone may be inconsistent, because the temperature distribution in the gasification chamber is not consistent.